The present invention relates to a cathodic arc deposition method and apparatus. More particularly a cathodic arc deposition method and an apparatus, which generates a plasma of electrically conducting materials for the application of coatings to surfaces of a substrate by way of condensation of evaporated material. This apparatus, which includes means for separating source material macroparticles from the ion stream, can be used for forming a high quality wear resistant coating on cutting tools, forming tools and mechanical components and alike.
Cathodic arc deposition principally includes generating a vapor emission of a film forming material from an evaporation source (cathode) by arc discharge in a vacuum chamber, and depositing the vapor on a substrate under the application of a negative bias voltage. One or more arc spots, where the arc discharge is focused, are formed on the surface of the evaporation source, which is the cathode in an arc discharge circuit. Typical arc currents range between 50 to 500 amperes, with voltages between 15 to 50 volts. The arc plasma discharge conducts electric current between a cathode and an anode through the plasma created by vaporization and ionization of the target material by the arc. The target is vaporized by a low voltage arc plasma discharge in a vacuum chamber, which has been evacuated to a typical background pressure of at least 0.01 Pascal. The cathode (negative electrode) is an electrically isolated source structure, which is at least partially consumed during the process. The consumable portion of the cathode is called the “target” and is fabricated as a replaceable element clamped to a cooled, non-consumable element called the cathode body. The anode (positive electrode) may be an electrically isolated structure within the vacuum chamber or may be the vacuum chamber itself, and is not consumed during the process.
From an arc spot, ions, neutral atoms, macroparticles of source material, and electrons are emitted in a beam due to the high temperature at the arc spot. These ionized particles are emitted preferably perpendicular to the cathode target surface. Ions of source material, which form a plasma together with the emitted electrons, are the species of primary importance in the film deposition. One characteristic feature of cathodic arc deposition is that the energy of the incident evaporated ions is high enough to produce a high-density film with excellent hardness and wear resistance. In case of carbon evaporation, according to U.S. Pat. No. 5,799,549 of Decker, the invention has particular utility for forming a very hard and rigid coating of high aspect ratio on very thin cutting edges of razor blades. More than that, the rapid film formation and high productivity of the technique have been industrial application.
An undesirable side effect of the vaporization of the target material at the arc spot is the generation of droplets of molten target material, which are ejected from the target by reaction forces due to expansion of the vapor at the arc spot. These droplets, often called macroparticles, typically range in diameter from sub-micron to tens of microns. The droplets travel outward from the cathode surface at such velocities that they often become imbedded in the coating when they land on the substrate to be coated. Thus, cathodic arc coatings are often contaminated with macroparticles that adhere to the substrate surface, or leave holes where they once clung but have since been removed. The adhering macroparticles increase the friction coefficient between the coated work piece and the contact partner. As a result, the soft macroparticles leave holes and these represent sites for corrosion to initiate or cracks to propagate.
Thus, there is a significant, continuing need for industrial methods and apparatus to prevent or reduce the deposition of macroparticles while forming uniform, adherent carbon or metal compound coatings on substrate surfaces.
Various strategies have been developed to decrease the number of macroparticles incorporated into the coating. There are generally two different strategies: a first category of apparatus using some form of electromagnetic field to control and accelerate the arc, thus reducing macroparticles generation, and a second category using a filtering apparatus between the cathode source and the substrate so as to transmit the ionized fraction to the substrate, but to block the molten droplets. Conventionally a filtering apparatus may be constructed that uses electromagnetic fields, which direct or deflect the plasma stream.
Because macroparticles are neutral, they are not influenced by the electromagnetic field. That is why filtering methods work efficiently by placing the substrate out of the line of sight of the cathode target surface, so that macroparticles do not land directly on the substrate whereas arc spot acceleration methods are generally simpler but do not completely eliminate the macroparticles presence. A filtering apparatus according to the second category can provide a plasma angled filtering duct between the cathode chamber and a coating chamber, wherein the substrate holder is installed out of the optical axis of the plasma source. Focusing and deflecting electromagnetic field around the apparatus thus direct the plasma stream towards the substrate, while the macroparticles, uninfluenced by the electromagnetic field, continue to travel in a straight line from the cathode. However, bouncing of macroparticles off the baffles in the duct can result in transmission of some portion of them through the filter to reach the substrate. Baffles, also called macroparticle firewall, within the chamber physically block neutral particles emanating from the arc source and filter region as mentioned in US 2009/0065350 A1 of Anders.
The arc spot acceleration methods of the first category are generally simpler than the filtering methods, but do not completely eliminate the macroparticles generation. Whereas the filtering methods can generally be more effective, they bring additional complexity to the apparatus and reduce tremendously its yield. Examples of efforts to reduce the number of macroparticles incorporated into the coating on the substrate by using some kind of a filtering apparatus between the cathode source and the substrate to transmit the charged ionized fraction of the emitted particles and to block the neutral particles are shown below.
Another example is described and illustrated in U.S. Pat. No. 5,435,900 and in US 2004/103845 A1 by Gorokhovsky for a “Filtered Cathodic Arc Deposition Method And Apparatus”. This mechanical filtering mechanism traps macroparticles by altering the path of the plasma stream out of the optical axis of the plasma source toward the substrate, and trapping macroparticles in a baffle disposed generally along the optical axis of the cathode. However there is no direct line-of-sight from the target material to the substrate holder. For this reason, the apparatus incorporates a plasma duct surrounded by the deflecting magnetic system, a plasma source and a substrate holder mounted in the coating chamber out of the optical axis of the plasma source, where the plasma source and the substrate holder are surrounded by the focusing electromagnets. However the distance between the target material and the substrate holder is much too large to ensure that a significant portion of the charged particles will reach the substrate. In our present apparatus, this distance has been minimized to overcome this issue and improve the system yield. Also our apparatus introduces use of rotary arc cathode consumable on its outer cylinder jacket in contrary to use of fixed, flat cathode as proposed in above mentioned patent.
In WO 2010/134 892 A1 a filtered cathodic arc deposition apparatus having a rotatable cathode is disclosed, but instead of using rotary arc cathode consumable on its outer cylinder jacket, arc cathode consumed on base of cylinder is used.
Use of a cylindrical plasma duct containing a 90 degree bend, with electromagnetic coils to create a solenoidal magnetic field through the duct, and with a circular arc evaporation cathode at one end of the duct and a substrate at the other end. Prior filtered cathodic arc apparatus have been based upon circular and flat or cylindrical cathode and filter geometry, generally limiting the field of applications because of their low transmission. Examples of elongated, cylindrical cathodes are included in U.S. Pat. Nos. 4,609,564 and 4,859,489 of Pinkhasov; U.S. Pat. No. 5,037,522 of Vergason; and U.S. Pat. No. 5,279,898 of Welty, all of which describe the use of an elongated cathode in the form of a cylinder or rod, and make use of the self-magnetic field of the arc current to force its motion along the length of the cathode. Welty teaches that macroparticles generation can be reduced by application of an additional axial magnetic field component to accelerate and control the arc motion. This is realized by connecting both ends of the cathode to an additional power supply, which delivers a current through the target material creating a circumferential magnetic field around the target to control the arc longitudinal motion. Our advantage is that the cathode is connected only to its head with an arc cable. In addition to that, the arc spot sensors detect efficiently the position of the arc spot and the cathode rotation helps keeping the arc spot in its optimum location.
U.S. Pat. No. 5,127,030 of Tamagaki and U.S. Pat. No. 5,317,235 of Treglio describe a straight cylindrical filtering duct with no bend, a circular cathode located at one end of the duct, electromagnetic coils to generate the solenoidal magnetic field through the duct and partially block the direct line of sight deposition from the cathode to the substrate. Plasma emitted by the cathode is focused by the electromagnetic field at the system optical axis. An arc confinement ring (so-called anode) to stabilize an arc is located around the target. A coil which resulting magnetic field is perpendicular to the target surface at the center, is compressing the magnetic lines to bring the charged particles toward the substrate. The arc spots burn on the round flat target (mostly to the outer area in the filtering mode) at a high velocity. The neutral macroparticles are not deflected by the electromagnetic field and are blocked by the coil duct. When coating over a large area is required, a substrate is arranged considerably apart from the coil to utilize a plasma flow that spreads along a magnetic field. However this decreases the apparatus growth rate enormously and limits its usage. The key point is the anode location. Unlike the previous two references where the anode surrounds the target material, the anode is located between the source of magnetic field and the substrate in our present case.
U.S. Pat. No. 5,292,944 of Sanders and WO 03/087425 A1 of Sathrum introduce cylindrically symmetric arc sources which operate in an arched field geometry. The emitted ions leave the surface radially and are reflected by an electromagnetic field. In the first reference, the charged particles have to deflect their trajectories by 90° and even 180° in the second reference which is not as favorable for the filter efficiency as a straight duct detailed hereafter.
None of the references of the prior art disclose a rotary arc cathode having an evaporable surface of cylindrical shape and using mostly external magnetic field in addition to an internal magnetic field to control the movement of the arc on the cathode surface, nor is a filtering duct having the external magnetic field source into the vacuum chamber used as extractor for charged particles and baffle for macroparticles. The anode position plays an important role in the output of this source since it is located between the magnetic field source and the substrate.
Filtered cathodic arc sources have the advantage that the stream of vapor of cathode material emitted from the source is fully ionized, unlike non-arc-based deposition methods such as evaporation and sputtering. The fully ionized vapor stream from a cylindrical cathode would allow greater control over the target utilization as well as the energy of the particles reaching the substrate for coating or ion implantation, and would increase the reactivity of the vapor in forming compounds with reactive gases in the system, or with the substrate directly.
The present invention realizes the benefits of a filtered cathodic arc (fully ionized vapor stream, elimination of splattered macroparticles) and the benefits of a cylindrical rotating arc cathode (uniform target evaporation and uniform deposition on the substrate using linear motion and arc spot position detection) with high throughput. Moreover the invention is a compact system that can upgrade the regular cylindrical rotating arc cathodes (LARC®) technology. An easy system accessibility and maintenance make the invention user-friendly, which could not be accomplished by the prior art.